Simon Emde

Optimally routing and scheduling tow trains for JIT-supply of mixed-model assembly lines

In recent years, more and more automobile producers adopted the supermarket-concept to enable a flexible and reliable Just-in-Time (JIT) part supply of their mixed-model assembly lines. Within this concept, a supermarket is a decentralized in-house logistics area where parts are intermediately stored and then loaded on small tow trains. These tow trains travel across the shop floor on specific routes to make frequent small-lot deliveries which are needed by the stations of the line. To enable a reliable part supply in line with the JIT-principle, the interdependent problems of routing, that is, partitioning stations to be supplied among tow trains, and scheduling, i.e., deciding on the start times of each tow train’s tours through its assigned stations, need to be solved. This paper introduces an exact solution procedure which solves both problems simultaneously in polynomial runtime. Additionally, management implications regarding the trade-off between number and capacity of tow trains and in-process inventory near the line are investigated within a comprehensive computational study.

Part logistics in the automotive industry: Decision problems, literature review and research agenda

With the ongoing trend of mass-customization and an increasing product variety, just-in-time part logistics more and more becomes one of the greatest challenges in today’s automobile production. Thousands of parts and suppliers, a multitude of different equipments, and hundreds of logistics workers need to be coordinated, so that the final assembly lines never run out of parts. This paper describes the elementary process steps of part logistics in the automotive industry starting with the initial call order to the return of empty part containers. In addition to a detailed process description, important decision problems are specified, existing literature is surveyed, and open research challenges are identified.

Optimally loading tow trains for just-in-time supply of mixed-model assembly lines

In todays mixed-model assembly production, there are two recent trends—namely, increasing vertical integration and the proliferation of product variety—that shift focus to an efficient just-in-time part supply. In this context, many automobile manufacturers set up decentralized logistics areas referred to as supermarkets. Here, small tow trains are loaded with parts and travel across the shop floor on specific routes to make frequent small-lot deliveries that are needed by the stations of the line. This article investigates the loading problem of tow trains, which aims at minimizing inventory near the line while avoiding material shortages given the limited capacity of the tow trains. An exact polynomial-time solution procedure is presented and interdependencies with production planning, that is, the sequencing problem of product models launched down the line, are investigated in a comprehensive computational study.

Balancing mixed-model assembly lines: a computational evaluation of objectives to smoothen workload

Mixed-model assembly lines are widely used in a range of production settings, such as the final assembly of the automotive and electronics industries, where they are applied to mass-produce standardised commodities. One of the greatest challenges when installing and reconfiguring these lines is the vast product variety modern mixed-model assembly lines have to cope with. Traditionally, product variety is bypassed during mid-term assembly line balancing by applying a joint precedence graph, which represents an (artificial) average model and serves as the input data for a single model assembly line balancing procedure. However, this procedure might lead to considerable variations in the station times, so that serious sequencing problems emerge and work overload threatens. To avoid these difficulties, different extensions of assembly line balancing for workload smoothing, i.e. horizontal balancing, have been introduced in the literature. This paper presents a multitude of known and yet unknown objectives for workload smoothing and systematically tests these measures in a comprehensive computational study. The results suggest that workload smoothing is an essential task in mixed-model assembly lines and that some (of the newly introduced) objectives are superior to others.

Scheduling the part supply of mixed-model assembly lines in line-integrated supermarkets

Line-integrated supermarkets constitute a novel in-house parts logistics concept for feeding mixed-model assembly lines. In this context, supermarkets are decentralized logistics areas located directly in each station. Here, parts are withdrawn from their containers by a dedicated logistics worker and sorted just-in-sequence (JIS) into a JIS-bin. From this bin, assembly workers fetch the parts required by the current workpiece and mount them during the respective production cycle. This paper treats the scheduling of the part supply processes within line-integrated supermarkets. The scheduling problem for refilling the JIS-bins is formalized and a complexity analysis is provided. Furthermore, a heuristic decomposition approach is presented and important managerial aspects are investigated.

Cooperative twin-crane scheduling

This paper treats the crane scheduling in a container port where two cooperative gantry cranes (denoted as twin cranes) jointly store import containers arriving from the seaside in a storage yard. We aim to minimize the makespan while non-crossing constraints among cranes need to be considered and preemptive container moves are allowed, i.e.,źthe seaside crane sets down a container in some intermediate position where the landside crane takes over and delivers the container to its final storage position in the yard. Elementary complexity proofs are provided and efficient heuristic solution procedures are introduced and tested.

The berth allocation problem with mobile quay walls: problem definition, solution procedures, and extensions

The berth allocation problem (BAP), which defines a processing interval and a berth at the quay wall for each ship to be (un-)loaded, is an essential decision problem for efficiently operating a container port. In this paper, we integrate mobile quay walls into the BAP. Mobile quay walls are huge propelled floating platforms, which encase ships moored at the immobile quay and provide additional quay cranes for accelerating container processing. Furthermore, additional ships can be processed at the seaside of the platform, so that scarce berthing space at a terminal is enlarged. We formalize the BAP with mobile quay walls and provide suitable solution procedures.

The basic train makeup problem in shunting yards

In shunting yards, railcars of incoming trains are uncoupled and reassembled to outbound trains. This time-critical process that employs a complex system of switches, hump hills, and classification tracks requires plenty interdependent decision problems to be solved. An elementary decision task among these is the train makeup problem, which assigns railcars of inbound freight trains to outbound trains, such that the priority values of the selected cuts of railcars are maximized and given train capacities are observed. This assignment decision is further complicated by the fact that railcars cannot facultatively be selected, but the buildup sequences of incoming trains need to be considered. This work introduces and discusses the basic train makeup problem, analyses its complexity status and develops suited exact and heuristic solution procedures that are tested in a comprehensive computational study.

Berth allocation in container terminals that service feeder ships and deep-sea vessels

This paper treats a berth allocation problem (BAP) in dedicated container terminals where feeder ships and container vessels are jointly served. When assigning quay space and a service time to each calling ship particular focus is put on the container exchange between feeder ships and mother vessels, so that the weighted number of containers delivered by feeder missing their intended mother vessel (and vice versa) does not exceed a given upper bound. The resulting BAP is formalized, complexity proofs are provided, and suited optimization procedures are presented and tested.

Scheduling shipments in closed-loop sortation conveyors

At the very core of most automated sorting systems— for example, at airports for baggage handling and in parcel distribution centers for sorting mail—we find closed-loop tilt tray sortation conveyors. In such a system, trays are loaded with cargo as they pass through loading stations, and are later tilted upon reaching the outbound container dedicated to a shipment’s destination. This paper addresses the question of whether the simple decision rules typically applied in the real world when deciding which parcel should be loaded onto what tray are, indeed, a good choice. We formulate a short-term deterministic scheduling problem where a finite set of shipments must be loaded onto trays such that the makespan is minimized. We consider different levels of flexibility in how to arrange shipments on the feeding conveyors, and distinguish between unidirectional and bidirectional systems. In a comprehensive computational study, we compare these sophisticated optimization procedures with widespread rules of thumb, and find that the latter perform surprisingly well. For almost all problem settings, some priority rule can be identified which leads to a low-single-digit optimality gap. In addition, we systematically evaluate the performance gains promised by different sorter layouts.